Joist/girder evaluation for RTU
Joist/girder evaluation for RTU
(OP)
When doing an evaluation of the existing girders and joists when adding an RTU unit on a roof, what method do you use to check the existing joists? Do you use the point loads on the joists from the corner of the curb? Or do you convert the unit weight into a distributed load and add that to the joists? Regardless I will reinforce the joist webs at the corners of the curb. Another question regarding two curb corners that don't coincide with a joist...do you run angles between the joists to carry the curb load to the joists?
Thanks
Thanks






RE: Joist/girder evaluation for RTU
PE, SE
Eastern United States
"If a builder builds a house for someone, and does not construct it properly, and the house which he built falls in and kills its owner, then that builder shall be put to death!"
~Code of Hammurabi
RE: Joist/girder evaluation for RTU
M.S. Structural Engineering
Licensed Structural Engineer and Licensed Professional Engineer (Illinois)
RE: Joist/girder evaluation for RTU
Without the angle frame, a concentrated load on the deck could cause it to deflect and pull the waterproofing apart between the deck and the curb (since the curb is stiffer, and because it doesn't see the loading on the deck and therefore isn't moving.)
RE: Joist/girder evaluation for RTU
RE: Joist/girder evaluation for RTU
RE: Joist/girder evaluation for RTU
Dik
RE: Joist/girder evaluation for RTU
RE: Joist/girder evaluation for RTU
Not that it is a lot different, but I would convert the approximate point load into each joist into an equivalent MOMENT, and then add that to the code LL and in-situ DL MOMENT. There isn't usually any wiggle room in the LL but maybe they over-designed a bit for the actual DL.
I have used inverted channels, but usually they cut a hole in the deck and install angles and the curb, and then reflash. Or you can use small pipes or tubes and build a small rack. Depends on what is going on in the roof and what the owner will let you do.
RE: Joist/girder evaluation for RTU
I find it does not make much difference whether you use conc. loads or uniform loads. If you do NOT have corner loads from the RTU vendor, I would suggest that you the RTU weight since it is generally heavier at one end than the other.
Don't forget to add snow on top of the unit and account for drifting around it. Anchorage for wind uplift is also very important.